The present invention generally relates to the removal of dissolved gases from liquids. More particularly, the present invention relates to a process for removing an undesirable dissolved gas from a liquid by absorbing a preferred gas into a pressurized liquid, and subsequently lowering the pressure of the liquid to remove at least a portion of the undesirable and preferred dissolved gases.
Unwanted or undesirable gases are often found in liquids. For example, hydrogen sulfide gas is often found in industrial waste water streams, which is caustic and an environmental pollutant. In other instances, naturally occurring dissolved gases, such as oxygen, are found in processing liquids. For example, in circuit board processing liquids, the presence of oxygen can be detrimental to the overall process. Thus, an inert gas, such as argon, would preferably be present in the processing liquid as such inert gas would not adversely effect the circuit board processing.
Conventional practice for removing undesirable dissolved gases from a liquid has included the steps of placing the liquid within a tank so as to pressurize it. Desirable or preferred gas is entrained within the liquid to a certain extent due to the pressurization. The liquid is often stirred in an attempt to entrain additional desirable or preferred gas. However, such methods are only capable of entraining approximately five to seventeen parts per million of the desired dissolved gas into the target liquid. The liquid is then subject to a low pressure, such as a vacuum chamber, in order to remove at least a portion of the preferred gas and the undesirable gas. These gases are then passed through a scrubbing device, like a carbon filter, or the like.
The hydrocyclone has been used as a separator of particles with different specific gravities for years. In an effort to reduce bubble residence time, a variation of the general hydrocyclone, the air-sparged hydrocyclone described in U.S. Pat. No. 4,279,743 utilizes a combination of centrifugal force and air sparging to remove particles from a fluid stream. The stream is fed under pressure into a cylindrical chamber having an inlet configured to direct the fluid stream into a generally spiral path along a porous wall. The angular momentum of the fluid generates a radially directed centrifugal force related to the fluid velocity and the radius of the circular path. The porous wall is contained within a gas plenum having gas pressurized to permeate the porous wall and overcome the opposing centrifugal force acting on the fluid.
In operation, the unit receives and discharges the rapidly circulating solution while the air permeates through the porous wall. Air bubbles that emit from the wall are sheared into the fluid stream by the rapidly moving fluid flow. The invention claims that micro-bubbles formed from the shearing action combine with the particles or gases in the solution and float them toward the center of the cylinder as froth in a vortex. The centrally located froth vortex is then captured and exited through a vortex finder disposed at the upper end of the cylinder while the remaining solution exits the bottom of the cylinder.
One variation in the general ASH construction, as described in U.S. Pat. Nos. 4,838,434 and 4,997,549, includes employing a froth pedestal at the bottom of the cylinder to assist directing the froth vortex through the vortex finder. Another ASH modification includes replacing the vortex finder and froth pedestal with a fixed splitter disposed at the bottom of the cylinder and having a cylindrical knife-edge. The edge is positioned to split the helically flowing solution into components dependent upon the specific gravity of the components.
However, it has been found that traditional hydrocyclones, including air-sparged hydrocyclones, essentially stratify the various components of the contaminated fluid according to specific gravity. Accordingly, it is an object of the present invention to provide a modified hydrocyclone which becomes a mixer in order to inject desirable gas into the fluid, and subsequently eliminate fractionated or proportional amounts of desirable and undesirable or otherwise unwanted gases. It has been found that the present invention is capable of removing undesirable dissolved gases from a liquid in a more simple manner and to a greater degree than prior systems.
Accordingly, there is a continuing need for a method for removing undesirable dissolved gas from a liquid in a more simple manner and to a greater degree. The present invention fulfills these needs and provides other related advantages.
The present invention resides in a process for removing an undesirable dissolved gas from a liquid by injecting or otherwise entraining and absorbing a preferred gas into the liquid, and subsequently removing at least a portion of the dissolved undesirable and preferred gases from the liquid. This is done by overentraining the preferred gas in a elevated ratio to the undesirable or bad gas, for example 1-to-100. A percentage of the total undesirable gas is then removed through the delivered adjustment of the ratio of undesirable to desirable gas. This is done by degassing the liquid after treatment, so that the amount of the undesirable gas is significantly reduced, in the provided example 1/100th of the undesirable gas is present in the liquid after treatment.
The process comprises the steps of first pressurizing the liquid containing the undesirable dissolved gas. A hydrocyclone is selected having an inlet aperture size and configuration corresponding with the liquid pressure to optimize the liquid particle movement through a liquid vortex stream created by the hydrocyclone. A barrel diameter and length of the hydrocyclone is also optimized for liquid particle movement. The liquid containing the undesirable dissolved gas is then directed into the hydrocyclone to form a liquid vortex stream having an evacuated central area. A preferred gas is injected into the evacuated central area for absorption into the liquid vortex stream.
The liquid is then directed from the hydrocyclone into a diffusion column, and then into a pressure chamber having an upper gas region and a lower liquid region to collect large bubbles and non-dissolved preferred gas from the liquid. The preferred gas in the gas region of the pressure chamber is then directed back into the evacuated area of the liquid vortex so as to be recycled. Pressurized preferred gas is added into the pressure chamber when the gas level of the pressure chamber falls below a predetermined level.
The liquid which exits the pressure chamber and contains the absorbed preferred gas and undesirable dissolved gas at the new pressurized ratio has its pressure lowered to cause a proportional release of the absorbed preferred gas and the undesirable dissolved gas from the liquid. This may be done by passing the liquid through a cavitation plate and then directing the liquid into a discharge or de-gas tank. Preferably, the liquid is directed through an outlet pipe of increasing diameter and having an outlet thereof directed towards a liquid surface of the discharge tank. Alternatively, the liquid is directed through a vacuum cyclone device disposed within the discharge tank and having an outlet thereof directed towards a liquid surface of the discharge tank, or towards a flow reflector within the discharge tank.